1. Field of the Invention
The present invention relates to a laser oscillation frequency stabilizer which controllably locks the oscillation frequency of a tunable laser source such as a semiconductor laser by making use of saturated absorption spectra of atoms or molecules.
2. Detailed Description of the Related Art
The oscillation frequency of a laser light source portion such as of a semiconductor laser device significantly depends on the temperature of the laser light source portion and the current allowed to flow through the laser light source portion. There is such a problem as the oscillation frequency varies due to slight changes in temperature of the laser light source portion and in current flowing therethrough, so that the laser device cannot easily provide stabilized oscillation frequencies.
For this reason, various types of laser oscillation frequency stabilizers have been proposed to stabilize the oscillation frequency of the laser light source portion of laser devices. As a typical technique for laser oscillation frequency stabilizers, such a technique which makes use of absorption spectra of atoms or molecules is known, for example, those using the absorption spectra of atoms or molecules as the reference.
Among them, a laser oscillation frequency stabilizer that uses saturated, absorption spectra of atoms or molecules as the reference can obtain a spectral line width narrower than a linear absorption spectral line width that is broadened due to the Doppler effect. Thus, the oscillation frequency of the laser light source portion can be stabilized with high sensitivity.
The laser oscillation frequency stabilizer for stabilizing the oscillation frequency of the laser light source portion with saturated absorption spectra of atoms or molecules used as the reference, the following method is employed. That is, first, a laser beam (which is called xe2x80x9cpumping lightxe2x80x9d) having intensity enough to saturate the light absorption is introduced into an absorption cell so that the amount of the transmitted beam of light is detected by means of a first light-receiving device. At the same time, part of the transmitted beam of light that has passed through the absorption cell is reflected. Then, the reflected feeble laser light (which is called probe light) is introduced again into the absorption cell from the opposite direction. Then, the amount of the transmitted light that has been introduced into the absorption cell and passed therethrough is detected by means of a second light-receiving device. Thus, the oscillation frequency of the laser light source portion is controllably locked to a saturated absorption spectrum of a narrow line width in accordance with the light reception outputs of the two light-receiving devices.
FIG. 10 is an explanatory view showing one example of the conventional laser oscillation frequency stabilizer. In FIG. 10, reference numeral 1 designates a laser light source portion. The laser light source portion 1 generally includes a laser diode 2, a thermistor 3, a Peltier-effect device 4, and a plate heat radiator 5. The temperature of the laser diode 2 is controlled by means of a temperature control circuit 6.
A laser beam emitted from the laser diode 2 is directed to a condensing lens 7. Then, the beam is transmitted from an optical isolator 7A to be introduced into a polarization beam splitter 8. The laser beam is linearly polarized. The polarization beam splitter 8 reflects laser beams having components linearly polarized in a certain direction and transmits those linearly polarized in the direction orthogonal to that direction.
The linearly polarized laser beam that has passed through the polarization beam splitter 8 is guided into a quarter wavelength plate 9 to be circularly polarized. Then, the circularly polarized laser beam is introduced into a saturated absorption cell 10 as pumping light. In the saturated absorption cell 10, sealed are gaseous atoms and/or molecules, which have absorption spectra at certain wavelengths.
The saturated absorption cell 10 is provided with electromagnets 11. The magnetic. fields created by the electromagnets 11 are modulated by means of an oscillator 12. A transmitted circularly polarized laser beam that has passed through the saturated absorption cell 10 passes through an ND filter 13 and then guided into a half mirror 14. Part of the laser beam is reflected by the half mirror 14 in the direction opposite to that of travel, whereas the remainder of the laser beam passes through the half mirror 14 to be received by a first light-receiving device 15. The laser beam that is reflected by the half mirror 14 and travels in the opposite direction passes again through the ND filter 13 to be allowed into the saturated absorption cell 10 as feeble probe light. Then, the laser beam passes through the saturated absorption cell 10 to be guided into the quarter wavelength plate 9, where the laser beam is linearly polarized in the direction orthogonal to that of the original linearly polarized laser beam. This linearly polarized laser beam is guided into the polarization beam splitter 8 and then reflected by a polarized beam splitting plane 8a to be received by means of a second light-receiving device 16.
The light reception outputs of the first light-receiving device 15 and the second light-receiving device 16 are inputted to a divider 17. The divider 17 is adapted to divide the light reception output of the second light-receiving device 16 by that of the first light-receiving device 15. The division output from the divider 17 is inputted to a lock-in amplifier 18, which in turn detects the division output in synchronization with the oscillation output of the oscillator 12 to output the lock-in signal to a current control circuit 19. In accordance with the lock-in signal, the current control circuit 19 is adapted to control parameters, having wavelength dependency, such as LD injection current for locking the wavelength of the laser diode 2 to a wavelength of absorption spectra.
However, the laser oscillation frequency stabilizer of the prior art is adapted to Zeeman-modulate a saturated absorption spectrum and therefore has to be provided with electromagnets, a power source, and an oscillator of its own. This presents a problem of increasing the laser oscillation frequency stabilizer in size. In addition to this, the stabilizer also present another problem that the electromagnets generate heat to cause the laser diode 2 to increase in temperature and thus the laser diode 2 requires much power for controlling the temperature, thereby making it difficult to save power consumption.
The present invention was developed in view of the aforementioned circumstances. An object of the present invention is to provide a laser oscillation frequency stabilizer that can be reduced in size without deteriorating the accuracy of wavelength stability and can reduce power consumption.
According to the present invention as set forth in claim 1, the laser oscillation frequency stabilizer is characterized by comprising a laser light source portion having a laser light source of which oscillation frequency can be controlled and for emitting a laser beam; a polarized beam splitter portion for splitting a laser beam from the above-mentioned laser light source portion into a first laser beam and a second laser beam, the above-mentioned laser beams having linearly polarized components orthogonal to each other; a quarter wavelength plate for converting the above-mentioned two laser beams, split by means of the above-mentioned polarized beam splitter portion, into laser beams circularly polarized in directions opposite to each other; an absorption cell which is disposed in an optical path of the above-mentioned circularly polarized laser beams and in which gaseous atoms or molecules with a certain absorption spectrum are sealed and to which a uniform magnetic field is applied; a half mirror for reflecting partially each of the above-mentioned first laser beam and the above-mentioned second laser beam, which have passed through the abovementioned absorption cell, in the direction of incidence and in the direction opposite thereto; a first light-receiving portion for receiving the first laser beam having passed through the above-mentioned half mirror; a second light-receiving portion for receiving the second laser beam having passed through the above-mentioned half mirror; a third light-receiving portion for receiving the first laser beam having been reflected by means of the above-mentioned half mirror and having passed through the above-mentioned absorption cell; a fourth light-receiving portion for receiving the second laser beam having been reflected by means of the above-mentioned half mirror and having passed through the above-mentioned absorption cell; and a control portion for controllably locking the oscillation frequency of the above-mentioned laser light source to the above-mentioned absorption spectrum in accordance with light reception outputs provided by the above-mentioned first to fourth light-receiving portions.
The laser oscillation frequency stabilizer described in claim 2 is characterized in that, the above-mentioned control portion in claim 1 comprises a first divider for operating a ratio between light reception output of the above-mentioned first light-receiving portion and light reception output of the above-mentioned third light-receiving portion; a second divider for operating a ratio between light reception output of the above-mentioned second light-receiving portion and light reception output of the above-mentioned fourth light-receiving portion; a subtracter, into which output of the above-mentioned first divider and output of the above-mentioned second divider are inputted, for outputting a difference therebetween as an error signal; and a current control circuit for controlling current in accordance with the error signal of the above-mentioned subtracter so that an oscillation frequency of the above-mentioned laser light source coincides with the above-mentioned absorption spectrum.
The laser oscillation frequency stabilizer described in claim 3 is characterized in that, the above-mentioned polarized beam splitter portion in claim 1 comprises a first polarized beam splitting plane for splitting a laser beam incident on the above-mentioned polarized beam splitter portion into a laser beam due to a first linearly polarized component and a laser beam due to a second linearly polarized component, for transmitting the laser beam due to the first linearly polarized component, and for reflecting the laser beam due to the second linearly polarized component; and a second polarized abeam splitting plane for reflecting the laser beam due to the second linearly polarized component reflected by the above-mentioned first polarized beam splitting plane and for transmitting a laser beam due to a linearly polarized component in a direction orthogonal to the second linearly polarized component.
The laser oscillation frequency stabilizer described in claim 4 is characterized in that, the above-mentioned polarized beam splitter portion in claim 1 comprises a polarized beam splitting plane for splitting a laser beam, emitted from the above-mentioned laser light source portion and incident on the above-mentioned polarized beam splitter portion, into a laser beam due to a first linearly polarized component and a laser beam due to a second linearly polarized component, for transmitting the laser beam due to the first linearly polarized component, and-for reflecting the laser beam due to the second linearly polarized component; and a total reflective plane for reflecting the laser beam reflected by the above-mentioned polarized beam splitting plane.
The laser oscillation frequency stabilizer described in claim 5 is characterized in that, the above-mentioned polarized beam splitter portion in claim 1 is made of a birefringence substance for splitting a laser beam, emitted from the above-mentioned laser light source portion and incident on the; above-mentioned polarized beam splitter portion, into normal light or a laser beam due to a first linearly polarized component and abnormal light or a laser beam due to a second linearly polarized component, for transmitting the laser beam due to the first linearly polarized component, and for refracting and then transmitting the laser beam due to the second linearly polarized component.
According to the present invention, a stabilizer can be reduced in size without deteriorating the accuracy of wavelength stability and can reduce power consumption.